Veteran Montgomery County Council member Neal Potter entered the race for county executive last night, saying that his unhappiness with the rapid pace of growth in the county prompted him to launch an eleventh-hour challenge of fellow Democrat Sidney Kramer.

Potter, 75, whose 20 years on the council mark the longest tenure in county history, announced his candidacy at the board of elections at 7 p.m., two hours before the filing deadline.

"The issue is do you want more development," Potter said to a group of reporters who had gathered at the board's Rockville headquarters after rumors surfaced that he would seek the county's top elected post.

Potter said his decision to enter the Sept. 11 primary was a difficult one, but that he was continually pressured by people dissatisifed with Kramer's policies and concerned that voters have a viable choice.

Until yesterday, Henry Bain, a former council aide, was the only Democrat to take on Kramer. State's Attorney Andrew L. Sonner and Friendship Heights Village Council Chairman Alfred Muller toyed with running but concluded they couldn't overcome Kramer's incumbency and fund-raising abilities. Potter initially had encouraged Bain's candidacy, but suggested last night that Bain would not continue his campaign. Candidates have until July 12 to withdraw from the race.

Potter's political plans have taken some surprising turns this year. After saying he would run for reelection to the council on an incumbent slate with Kramer, Potter in January announced his retirement. At that time, he criticized the "bossism" and "real estate influences" that he said he believes have come to dominate county government.

Potter also had said his wife's health problems were a factor in his decision. Last night, Potter said his wife's health is improving. He said he has the energy and stamina to run a vigorous campaign and, if elected, to serve a four-year term.

Kramer said he looks forward to comparing his record to Potter's. He said he has worked cooperatively with members of the council, the legislature and the governor. Potter's tenure, Kramer said, has been one of isolation and of being at odds with other elected officials.

Potter's big challenge may be to overcome Kramer's war chest, which totals about $150,000. Potter said he needs to raise $50,000, an amount that he estimated would be about one-tenth of the total Kramer will ultimately raise.

Chevy Chase businessman Al Ceccone is the only Republican candidate for county executive.